Long story short, the Guardian article is jumping the gun a bit! :)
"Root cause" in particular is silly; notice this sentence from the article: Professor Carrell said changes in the placenta during pregnancy alter how much oxygen the growing baby receives, but this can trigger the release of free radicals that oxidise angiotensinogen and cause blood pressure to rise.
So changes in the placenta come first -- and the quoted researcher was careful to point this out. And what they do is trigger biological cascades, one of which makes it much easier to break angiotensinogen -- which is a precursor molecule to some molecules that raise bp -- apart into the molecules that raise bp.
But the sFlt-1 and sEng molecules are actually acting first, and poking the holes in the vasculature that trigger the release of the angiotensinogen.
Re: ACE inhibitors -- the development of the placenta is intertwined with the development of the fetus and the development of blood vasculature. If it alters the course of placental development it can also alter the course of fetal development -- and boom, birth defects. I don't expect anyone to start scripping ACE inhibitors, not if they could block sFlt-1 instead.
I'll have more up after I finish reading the paper and brush up a bit on my redox reactions!